Apparatus for treating articles coated with concentrated aqueous solutions of gold cyanide



-prl 14,-1942. R MiNER 2y 79,580

v. ,2 APPARATUS FOR TREATING ARTICLES COATED WITH A CONCENTRATED AQUEOUS SOLUTIONv OF GOLD CYANIDE Filed Dec. 9, 1939 V r5.7 faw.

from? I Patented pr. 14, 1942 APPARATUS FOR. TREATING ARTICLES COATED WTH CONCENTRATED AQUEOUS SOLUTIONS OF GOLD CYANIDE Robert V. Miner, Chicago, Ill., assigner to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application December, 1939, Serial No. 368,398

(Cl. 26S-4Z) l Claim.

This invention relates to electroplating, and more particularly to the electroplating of articles with precious metals such as gold.

An object of this invention is the provision of improved methods and apparatus for producing plated coatings of gold and other precious metals on articles with a minimum loss of precious metal.

it is customary to electroplate articles in an electrolytic solution of the desired coating metal and then wash the articles in water to remove the plating solution therefrom. This practice results in the steady withdrawal or drag-out of solution, with the contained metal, from the plating bath on the surfaces of the articles. This prospective loss of metal is not serious in the usual plating process where relatively inexpensive metal solutions are employed, but when gold or other precious metals are used in the plating bath, the value of the withdrawn solution mounts rapidly. Consequently, means have been employed to partially recover metal from the drag-out solution in precious metal plating operations.

Under one former practice the plated articles were Washed successively in a series of still water tanks and the metal Was extracted from the wash water at intervals. The metal content of the wash tanks was built up progressively, with the smallest metal content in the last tank, which determined the minimum metal loss. The rate of metal loss was progressive, as the metal content of the last wash tank increased steadily with use. It was diicult to hold this value to desirably low levels because the metal content changed constantly and also because of practical limitations in the number of tanks that could be used in the series and the necessity for interrupting the entire process for any changes in the tank arrangements. Due to the handling and processing involved, it was considered necessary to have a fairly high gold content in the preliminary wash tanks, with a relatively high metal content in the iinal tanks, to justify the metal recovery operations. Another objection to the former practice was the necessity for a final cleaning operation on some parts. For example, on certain parts plated in gold cyanide it is necessary to remove all traces of the plating solution, Because the final wash tank in the series contained some, and an unknown quantity of plating solution, these parts received a final washing in running water.

In accordance with one embodiment of this invention, a process and apparatus are provided Cil for electrodepositing a coatingfoi gold on an article with a minimum loss of gold. The apparatus comprises a plating tank and two tanks containing wash water adjacent thereto in which the plated parts are immersed successively. Wash water is withdrawn continuously from the iirst tank, circulated through a series of cylinders containing zinc turnings which precipitate and extract the gold, and then discharged into the second wash tank. An overflow is provided between the first and second tanks so that the rst tank is constantly supplied with water from the second tank. In this manner the wash water is constantly recirculated and the gold content in the second wash tank, which determine the drag-out loss, is maintained at a very low value.

Following is a more detailed description of the invention, taken in conjunction with the appended drawing, in which the single figure is a sche-y matic drawing of an apparatus incorporating certain features of the invention.

In this apparatus and method a conventional plating tank lll provided with the usual electrodes and power supply (not shown) is employed. The tank contains plating solution, such as a sodium gold cyanide solution and the articles are plated while suspended in the solution in accordance with the usual practice.

Immediately adjacent to the plating tank are two open top wash tanks Il and I2 in which they articles are washed successively after they are removed from the plating solution. The Wash tanks can be made of various easily formed materials, such as wood, concrete or metal and are conveniently made in one body with an intermediate partition l3 between them to form the first or initial wash tank H and the second or iinal wash tank I2.

At the beginning of the process both wash tanks are lled with distilled water. As more and more articles are removed from the plating tank and immersed in the wash tanks the total quantity of gold cyanide flushed from the surfaces of the articles into the Wash water in the initial wash tank tends to build up rapidly. At the Sametime the gold cyanide content of the nal or second wash tank also tends to increase, but in smaller unit quantities, due to drag-out from the rst wash tank. Ordinarily, any dragout from the second wash tank would be lost and in order to minimize this loss, means vare provided for removing the gold from the wash water as rapidly as it is deposited therein.

Wash water is withdrawn continuously from the rst Wash tank I I through a pipe I4 having a. manual control valve I5 therein and connected to an opening near the bottom of the tank. 'Ihe wash water is withdrawn by means of a suitably driven pump I6 connected to the pipe.

The pump forces the Wash water through a pipe I1 into a lower header I8. Extending from the lower header are ve spaced branch pipes I9, 20, ZI, 22 and 23, each having a manual control valve 24, 25, 26, 21 and 28 therein. The upper end of each branch pipe is connected into the bottom end of a hollow cylinder 29, 30, 3|, 32 and 33. Each of the Ve cylinders contains a quantity or batch of zinc turnings 34.

Connected into the top of each of the four cylinders on the left in the drawing (cylinders 29, 3U, 3I and 32) is a branched pipe with one branch thereof 35, 36, 31 and 38, under control of a manual valve 39, 40, 4l and 42, joined to the pipe running into the next adjacent cylinder to the right. The other branch 43, 44, 45 and 46 of each of these branched pipes contains a manual valve 41, 48, 49 and 50 and is connected into a common upper header 5I. The right hand cylinder 33 is also connected into the header through a pipe 52 with a Valve 53 therein, and with the left hand cylinder 29 through another pipe 54 with a manual valve 55 therein.

A pipe 56 connects the upper header with an inlet into the second or final wash tank I2. In the top portion of the partition I3 between the two wash tanks is an opening 51 through which wash water flows from the second Wash tank I2 to the first wash tank Il.

With the above described system of piping, valves and pumps, it is possible to withdraw the wash -water from the rst tank, circulate it through the complete series of cylinders beginning with any one of the cylinders, and then discharge the water into the second or final wash tank.

As the wash water is brought into contact with the zinc turnings in the cylinders, the gold is precipitated from the cyanide solution and deposited in the cylinders, from which it is removed periodically. The cylinders or tubes are easily dismantled for removal of the accumulated gold by removing a threaded cap on the tube into which the connecting pipes are joined.

For maximum gold extraction it is desirable to contact the solution with fresher zinc as the solution is weakened by partial gold extraction. To accomplish this, the Zinc is replaced in one cylinder at a time, in sequence and at uniform time intervals. For example, one cylinder is replenished one week, the next cylinder to the right, or the next in the series, is replenished one week later, and so on. Under this practice, the zinc in successive cylinders in the series is of graduated service age; The circulation of wash Water through the cylinders is controlled, by means of the valves in the interconnecting pipes, to bring the wash water in contact lirst with the oldest zinc, successively with zinc of lesser service age, finally with the freshest zinc just before the water is discharged into the iinal wash tank.

With the above described apparatus the loss of gold is very low. When plating with a sodium gold cyanide solution of the usual concentration the gold content of the rst wash tank may run around .013 ounce per gallon of water. By circulating this wash water through the extraction tubes or cylinders continuously the gold content of the second or final wash tank, which determines the drag-out loss, can be maintained consistently at a value as low as .O03 ounce per gallon of water. The resultant gold loss is insignificant compared with the rate of loss under former methods.

The above described apparatus and methods can be adapted for plating articles with other solutions and metals by merely using a suitable extracting agent for the selected metal or solution in the tubes or cylinders. It will be apparent that other modifications and adaptations of the methods and apparatus are feasible and it is to be understood that the invention is limited only by the scc-pe of the appended claim,

What is claimed is:

Apparatus for treating articles coated with a concentrated aqueous solution of gold cyanide such as is employed in a gold plating bath to remove said solution from the article surfaces and recover the gold from said solution, an initial wash tank containing a dilute4 aqueous solution of gold cyanide for receiving the articles coated with the concentrated solution and ushing the concentrated solution from the articles, said dilute solution having a maximum gold content of .013 ounce per gallon, a second wash tank containing nal wash water for flushing from the articles the dilute solution withdrawn on the articles from the initial wash tank, said final wash water having a maximum gold content of .003 ounce per gallon, means for withdrawing dilute solution from the initial wash tank at a uniform rate, means for supplying final wash water from the second tank to the initial tank at the ysame rate at which dilute solution is Withdrawn from the initial tank to maintain a constant quantity of solution in the initial tank, a series of containers each having zinc therein, the containers being connected in series and the zinc in said containers being of successively graduated age, a system of pipes interconnecting the two wash tanks and the series of containers, and a pump for circulating. all of the dilute solution withdrawn from the initial wash tank through the series of containers beginning with any selected container and nally delivering all of the treated solution to the second wash tank to maintain a uniform supply of nal wash water in the second tank.

- ROBERT V. MINER. 

